If $tau$ and $tau_1$, ..., $tau_n$ denote numbers, then the term
{tt (- $tau$ $tau_1 ... tau_n$)} denotes the difference between the
number denoted by $tau$ and the numbers denoted by $tau_1$ through
$tau_n$. An exception occurs when $n=0$, in which case the term
denotes the negation of the number denoted by $tau$.